Go to Settings → Connections, choose the connection, and remove it. That deletes the stored credential and that connection’s settings from your device.
There is no Air Widgets account to close — the only credential involved is your own Airtable access. You can also revoke the token or authorisation from inside Airtable, which cuts the app off immediately, whether or not you still have it installed.
Widgets that pointed at a removed connection will stop updating. Delete them, or edit them to point at a connection you still have.